The anterior pituitary hormone prolactin (PRL) is involved in reproductive and nonreproductive processes. Hyperprolactinemia is a prevalent disorder of pituitary function, and a common cause of infertility in women and impotence in men. PRL is the only anterior pituitary hormone whose secretion is tonically inhibited, and dopamine is accepted as the major physiological inhibitor of PRL secretion. Evidence suggests the existence of a prolactin- releasing factor (PRF), but its identity remains unknown. Using an anterior pituitary cell perifusion system, we recently showed that the posterior pituitary contains a potent PRF which causes a rapid, hormone-specific, concentration-dependent stimulation of PRL release. PRF is a small peptide which is distinct from known PRL secretagogues. We are presently isolating PRF from bovine posterior pituitaries. Thus far, the characterization of posterior pituitary PRF has relied on an in vitro bioassay only. The goal of this proposal is to establish an in vivo model for estimating PRF activity, which in conjunction with the in vitro bioassay, will be used throughout our purification efforts. The specific aims of this proposal are: 1. To establish an extraction procedure for PRF which is suitable for in vivo administration. Initially, rat and bovine posterior pituitaries will be extracted with acetic acid, filtered through 5K dalton membranes, and treated with performic acid to inactivate vasopressin and oxytocin. PRF activity in partially purified extracts will be verified in vitro prior to in vivo administration. 2. To identify the optimal conditions for the in vivo bioassay. The route of administration and the endocrine state of the animal will be tested to determine the most sensitive conditions. 3. To determine the characteristics of PRF activity in vivo. The dose-dependency and the effects of repeated injections of the extracts will be examined. The development of an in vivo bioassay will facilitate the isolation of PRF. Once the structure of PRF is known, alternative therapeutic approaches for the treatment of hyperprolactinemia with PRF antagonists may be developed.